CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/463,037 filed Feb. 11, 2011, incorporated by reference herein.
This application is also a continuation-in-part of U.S. patent application Ser. No. 13/374,439 filed Dec. 29, 2011 that is incorporated by reference herein. This application is also a continuation-in-part of U.S. patent application Ser. No. 13/136,331 filed Jul. 28, 2011 that claims the benefit of U.S. Provisional Patent Application Serial Nos. 61/400,504 filed Jul. 29, 2010, and 61/403,915 filed Sep. 23, 2010, all of which are incorporated by reference herein. This application is also a continuation-in-part of U.S. patent application Ser. No. 12/802,849 filed Jun. 15, 2010 that claims the benefit of the following U.S. Provisional Patent Application Ser. Nos. 61/268,708, filed Jun. 15, 2009; 61/270,754, filed Jul. 13, 2009; 61/336,911 filed Jan. 28, 2010; 61/395,564 filed May 14, 2010; 61/395,752 filed May 17, 2010; and 61/396,390 filed May 26, 2010; all of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION The present invention is directed to dynamic fixation assemblies for use in bone surgery, particularly spinal surgery, and in particular to longitudinal connecting members and cooperating bone anchors or fasteners for such assemblies, the connecting members being attached to at least two bone anchors.
Historically, it has been common to fuse adjacent vertebrae that are placed in fixed relation by the installation there along of bone screws or other bone anchors and cooperating longitudinal connecting members or other elongate members. Fusion results in the permanent immobilization of one or more of the intervertebral joints. Because the anchoring of bone screws, hooks and other types of anchors directly to a vertebra can result in significant forces being placed on the vertebra, and such forces may ultimately result in the loosening of the bone screw or other anchor from the vertebra, fusion allows for the growth and development of a bone counterpart to the longitudinal connecting member that can maintain the spine in the desired position even if the implants ultimately fail or are removed. Because fusion has been a desired component of spinal stabilization procedures, longitudinal connecting members have been designed that are of a material, size and shape to largely resist bending (flexion, extension and lateral), torsion, shear, distraction and compression, and thus substantially immobilize the portion of the spine that is to be fused. Thus, longitudinal connecting members are typically uniform along an entire length thereof, and usually made from a single or integral piece of material having a uniform diameter or width of a size to provide substantially inelastic rigid support in all planes.
An alternative to fusion, which immobilizes at least a portion of the spine, and the use of more rigid longitudinal connecting members or other rigid structure has been a “soft” or “dynamic” stabilization approach in which a flexible loop-, S-, C- or U-shaped member or a coil-like and/or a spring-like member is utilized as an elastic longitudinal connecting member fixed between a pair of pedicle screws in an attempt to create, as much as possible, a normal loading pattern between the vertebrae in flexion, extension, side bending, distraction, compression and torsion. Another type of soft or dynamic system known in the art includes bone anchors connected by flexible cords or strands, typically made from a plastic material. Such a cord or strand may be threaded through cannulated spacers that are disposed between adjacent bone anchors when such a cord or strand is implanted, tensioned and attached to the bone anchors. The spacers typically span the distance between bone anchors, providing limits on the bending movement of the cord or strand and thus strengthening and supporting the overall system. Shear forces are not well resisted by the typical cord and spacer stabilization systems. Such tensioned cord and spacer systems may also cause facet joint compression during spinal movement, especially flexion.
The complex dynamic conditions associated with spinal movement create challenges for the design of elongate elastic longitudinal connecting members that exhibit an adequate fatigue strength to provide stabilization and protected motion of the spine, without fusion, and that allow for some natural movement of the portion of the spine being reinforced and supported by the elongate elastic or flexible connecting member. A further challenge are situations in which a portion or length of the spine requires a more rigid stabilization, possibly including fusion, while another portion or length may be better supported by a more dynamic system that allows for protective movement.
SUMMARY OF THE INVENTION Longitudinal connecting member assemblies according to the invention for use between at least two bone anchors provide dynamic, protected motion of the spine and may be extended to provide additional dynamic sections or more rigid support along an adjacent length of the spine, with fusion, if desired. A disclosed embodiment of a dynamic longitudinal connecting member assembly according to the invention has an inner segment or core made from a cord being tensioned and fixed at least at either end of the assembly. The cord is received by at least one hard, rigid, inelastic segment or sleeve, the sleeve attachable to at least one bone anchor. Illustrated sleeves of the invention include a surface or surfaces for direct engagement with a compression insert that in turn directly engages a shank of a polyaxial bone screw. Such a sleeve may also be configured for direct engagement with a closure top that closes off the U-shaped channel of the insert as well as a U-shaped channel of the polyaxial bone screw portion that receives the insert and the sleeve. The sleeve includes outwardly extending portions for engaging and seating upon top arm surfaces of the insert, the sleeve portions sandwiched between the insert arms and the closure top, advantageously providing an even load from the closure top, to the sleeve, to the insert, and then to the bone screw shank. In some embodiments, the cord is received by at least a pair of such sleeves, each sleeve attachable to a bone anchor. In some embodiments, the sleeve or sleeves slidingly receive the cord. In other embodiments, the sleeve or sleeves are either fixed or left unfixed to the cord by the surgeon, resulting in a connecting member having variable segmental stiffness along a length thereof.
A variety of embodiments according to the invention are possible. Additional sleeves may be attached to additional bone anchors and cooperate with additional cut-to-length spacers to create longer assemblies. Sleeves may also be extended to provide inelastic rod, bar or tube extensions, especially on one end.
Spacers with the same or different measures of rigidity may be connected according to embodiments of the invention. Spacers of the invention may further be equipped with outer extending portions or nubs that are received by grooves or channels formed in upper or peripheral outer surfaces or flanges of the sleeves, such cooperation limiting movement of the spacer with respect to a respective adjacent sleeve, providing torsion control. Either rigid lengths or cords may be of greater or lesser lengths for attaching to one or a plurality of bone anchors. In some embodiments, longitudinal connecting member assemblies may be dynamically loaded before insertion, or after being operatively attached to at least a pair of bone anchors along a patient's spine by tensioning the inner cord and at least partially compressing an end bumper and/or at least one spacer located between the bone anchors. Typically, the at least one spacer has some flexibility in bending, with the spacer protecting and limiting flexing movement of the inner core.
An object of the invention is to provide lightweight, reduced volume, low profile assemblies for use with at least two bone anchors. Furthermore, it is an object of the invention to provide apparatus and methods that are easy to use and especially adapted for the intended use thereof and wherein the apparatus are comparatively inexpensive to make and suitable for use.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a set of longitudinal connecting member components according to the invention, in particular a first sleeve with parallel flanges with U-shaped cut-outs and opposed tubular extensions, a second sleeve with parallel flanges, one flange with a cut-out and one tubular extension, a third sleeve with parallel flanges with U-shaped cut-outs and elongated opposed tubular extensions, a fourth sleeve with parallel flanges, one flange with a cut-out and one elongated tubular extension, a fifth sleeve with parallel flanges with U-shaped cut-outs, one tubular extension and an opposed elongated tubular extension and a sleeve/rod coupler having a flange with U-shaped cut-out and integral tubular extension, all for use with bone anchor assemblies of the invention.
FIG. 2 is an enlarged perspective view of a longitudinal connecting member assembly of the invention shown utilizing the second sleeve and the sleeve/rod coupler of FIG. 1, and shown with a cord, closure tops (in exploded view), a spacer, an elastic bumper and a blocker, the connecting member assembly for use with bone screws such as that shown in FIG. 4, and with portions of the sleeves and cord located within the spacer and the bumper shown in phantom.
FIG. 3 is a front elevational view of the connecting member assembly of FIG. 2 with portions broken away to show the detail thereof (portions of the cord shown in phantom).
FIG. 4 is a reduced and partial perspective view of a polyaxial bone screw assembly shown assembled with the second sleeve of FIG. 1.
FIG. 5 is an enlarged and partial front elevational view of the assembly of FIG. 4 with portions broken away to show the detail thereof.
FIG. 6 is a reduced and partial perspective view of the bone screw assembly of FIGS. 4 and 5 further shown with a portion of a longitudinal connecting member assembly that includes a cord, a spacer (shown transparent), a bumper(shown transparent) and a blocker for securing the cord at an end of the connecting member assembly.
FIG. 7 is an enlarged perspective view of the spacer shown in FIGS. 2, 3 and 6.
FIG. 8 is a reduced and partial perspective view of three bone screw assemblies of FIG. 4, shown with a longitudinal connecting member including a cord, three sleeves, two spacers, one bumper and a blocker and shown in exploded view with cooperating closure tops.
DETAILED DESCRIPTION OF THE INVENTION As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. It is also noted that any reference to the words top, bottom, up and down, and the like, in this application refers to the alignment shown in the various drawings, as well as the normal connotations applied to such devices, and is not intended to restrict positioning of the connecting member assemblies of the application and cooperating bone anchors in actual use.
With particular reference to FIGS. 4-6 and 8, polyaxial bone screws, generally 1″ are shown cooperating with longitudinal connecting member assemblies of the present invention, that include sleeves, generally 804 of the present invention shown in FIG. 1. It is noted that the particular polyaxial bone screw assembly 1″ shown in FIGS. 4-6 and 8 is an example of a bone screw for use with the present invention and other types of bone screws, for example screws having top or bottom loaded shanks and/or different types of structure for capturing the shank within the receiver may be used. The components of the bone screw assembly 1″ and methods of assembly thereof are described in detail in applicants' U.S. patent application Ser. No. 13/374,439, filed Dec. 29, 2011 (hereafter identified as the '439 application) that is incorporated by reference herein. Briefly, and with particular reference to FIGS. 4 and 5, a few more details regarding the bone screw assembly 1″ that may be helpful are provided here: The assembly 1″ includes a shank 4 having a body 6 for insertion into a vertebra, and an upper portion or head 8; a receiver 10; a split or open retaining ring 12; a compression insert 14 and a closure top 18 or 18′. The shank head 8 has an outer partially spherical or curved surface, the head 8 connecting with the shank body 6 at a neck 26. The shank upper portion 8 is configured for a pivotable connection between the shank 4 and the retainer 12 prior to fixing of the shank 4 in a desired position with respect to the receiver 10. The receiver 10 is for the pivotable housing of the shank head 8 with respect to the receiver 10 in a cavity defined by a receiver base 60. The receiver 10 further includes a pair of upstanding arms 62 forming a U-shaped channel for openly receiving the insert 14, one of the sleeves 804 and the closure top 18 or 18′. Each arm 62 has substantially planar front and rear surfaces 69. The receiver is sized and shaped to mate with either the closure top 18 fully described in the '439 application that is utilized in the present invention as a cord slip or sliding closure, or with a cord fixing closure top 18′ that will be described in greater detail below. The split ring shank retainer 12 is for pop-on or snap-on capturing of the spherical shank head 8 within the receiver 10. The insert 14 includes upwardly extending arms having top surfaces 144, the arms defining a U-shaped channel for receiving a portion of the desired sleeve, generally 804. The illustrated pressure insert 14 is configured for independent locking by a tool or instrument, thereby allowing the pop-on polyaxial screw to be distracted, compressed and/or rotated along and around a longitudinal connecting member assembly, such as the assembly 801 shown in FIG. 8, to provide for improved spinal correction techniques. Such a tool engages the pop-on receiver 10 from the sides and then engages the insert 14 to force the insert down into a locked position within the receiver and against the shank head 8. With the tool still in place and the correction maintained, the connecting member assembly 801 is then locked within the receiver channel by a closure top 18 or 18′ followed by removal of the tool. This process may involve multiple screws all being manipulated simultaneously with multiple tools to achieve the desired correction.
In the '439 application, the bone screw assembly 1″ made up of the shank 4, receiver 10, retainer 12, insert 14 and closure top 18 is shown and described cooperating with a hard metal rod. In this application is shown and described that the bone screw assembly 1″ may also cooperate with soft or dynamic stabilization longitudinal connecting members that further include one or more sleeves, generally 804 (as shown in FIG. 1) and further with cooperating, spacers, bumpers and an inner tensioned cord, such as, for example, the connecting member, generally 801, shown in FIG. 8. Specifically, in FIGS. 2-6, for example, the bone screw 1″ is illustrated with one or more hard, inelastic, flanged sleeves, generally 804, through which a tensioned cord 806 extends. FIG. 1 illustrates a set of such sleeves 804 having different end configurations. With further reference to FIGS. 2, 3, 6 and 8, also illustrated is a cooperating cord blocker or fixer 810 with a cord fixing set screw 812. FIGS. 6 and 8 further illustrate an elastic tubular end bumper or spacer 814, and one or more elastic or inelastic spacers 816 having a somewhat trapezoidal profile, also illustrated in FIG. 7, that are each located about the cord 806 and are disposed between each pair of bone anchors 1″ of the overall assembly 801 illustrated in FIG. 8, for example. The cylindrical and tubular bumper 814 and non-cylindrical spacers 816 are illustrated as transparent in some of the drawing figures, allowing for viewing of the sleeves, generally 804, and the tensioned cord 806. However, it is foreseen that in other embodiments, the spacers and bumpers may be made of materials that may not be transparent or translucent.
Also as shown in FIGS. 2 and 3, two types of bone screw closures are utilized, either the slide or slipping closure top 18 or the cord gripping closure top 18′. The closure top 18 is substantially cylindrical and includes a an outer helically wound guide and advancement structure 182 in the form of a flange that operably joins with the guide and advancement structure disposed on the arms 62 of the receiver 10. The flange form utilized in accordance with the present invention may take a variety of forms, including those described in Applicant's U.S. Pat. No. 6,726,689, which is incorporated herein by reference. The illustrated closure structure 18 also includes a top surface 184 with an internal drive 186 in the form of an aperture that is illustrated as a star-shaped internal drive such as that sold under the trademark TORX, or may be, for example, a hex drive, or other internal drives such as slotted, tri-wing, spanner, two or more apertures of various shapes, and the like. A driving tool (not shown) sized and shaped for engagement with the internal drive 166 is used for both rotatable engagement and, if needed, disengagement of the closure 18 from the receiver arms 62. A base or bottom surface 188 of the closure is planar and further includes a rim 190 and may include a central point (not shown), the rim 190 and or point (not shown) for engagement and penetration into the respective sleeve, generally 804 as will be described in greater detail below. The closure top 18 may further include a cannulation through bore (not shown) extending along a central axis thereof and through the top and bottom surfaces thereof. Such a through bore provides a passage through the closure 18 interior for a length of wire (not shown) inserted therein to provide a guide for insertion of the closure top into the receiver arms 62. The top 18′ only differs from the top 18 in that the top 18′ does not include a bottom rim or bottom point, but rather a downwardly extending cord fixing portion 189′ for gripping the cord 806, the illustrated embodiment further having a narrow extension 190′ for penetrating into the cord 806. The slide or slip closure top 18 engages a respective sleeve 804 but not the cord 806, allowing the cord to slip or slide within the polyaxial screw 1″. The grip closure top 18′ extends through the cooperating sleeve 804 and grips and fixes the cord 806 against a surface of the sleeve 804 and thus fixes the cord 806 in relation to the respective polyaxial screw 1″.
With further reference to FIG. 8, for example, the polyaxial bone screws 1″ are shown with the dynamic stabilization longitudinal connecting member assembly 801 that includes a plurality of the hard, rigid, inelastic, flanged sleeves, generally 804 through which a tensioned cord 806 extends. The cord 806 is shown in phantom in FIGS. 2 and 3, that also illustrates a cooperating cord blocker or fixer 810 with a cord fixing set screw 812, an elastic end bumper 814, and an elastic or inelastic spacer 816 located about the cord 806 and located for placement between a pair of bone anchors 1″. The tubular bumper 814 and more angular spacers 816 are often shown in the drawings as transparent, allowing for viewing of the sleeves, generally 804, and the tensioned cord 806. However, it is foreseen that in other embodiments, the spacers 816 may be made of materials that may not be transparent or translucent. More details about the cords, blockers set screws and elastic bumpers that are the same or similar to the cord 806, blocker 810 and set screw 812 and elastic bumper 814 are described in detail in U.S. patent application Ser. Nos. 12/802,849 and 13/136,331, both of which are incorporated by reference herein.
With further reference to FIGS. 1, 6 and 8, for example, tubular extensions of some of the sleeves 804 may extend into and through the respective adjacent cooperating bumper 814 and/or spacer 816. Such spacer overlap with respect to the sleeves provides advantageous anti-shear support for the connecting member 801 shown in FIG. 8, for example. Also, as shown in FIG. 8, a portion of the cord blocker 810 may also extend into a bore of the bumper 814. The bumper 814 also extends about the cord 806 and is typically made from an elastomer while the other spacers 816, although very often elastomeric, may be made from a material with a different durometer, typically (but not always) being tougher and less compressible than the material of the bumper 814. The sleeves 804 and in some embodiments the spacers 816 are typically made from a hard, non-elastic material, such as a metal or metal alloy, for example, cobalt chromium. Flanged portions of the sleeves 804 are located on either side of the bone screw receivers 10, the flanges abutting against the spacers 816 or the bumper 814, the flanges extending radially outwardly to an extent to engage ends of adjacent spacers or the bumper, resulting in a stable, secure, substantially full contact between the individual elements of the assembly 801. Furthermore, the flanges allow for assembly and dynamic setting of the connector 801 prior to implantation, if desired, with the cord 806 being placed in tension and at least the bumper 814 being placed in compression. In some embodiments of the invention, tensioning of the cord 806 and compression of the bumper 814 and optionally the spacers 816 may be performed after the assembly 801 is attached to the bone screws 1″.
Sleeves 804 of the invention may be provided with or without tubular extensions, on one or both sides thereof, and with different lengths of tubular extensions, as shown, for example, in FIGS. 1 and 8. With particular reference to FIG. 1, six different types of sleeves 804 are shown. The illustrated sleeves include: a parallel flanged sleeve 804A having opposed tubular extensions; a parallel flanged sleeve 804B having a single tubular extension; a parallel flanged sleeve 804C having opposed elongate tubular extensions; a parallel flanged sleeve 804D having a single elongate tubular extension; a parallel flanged sleeve 804E having opposed tubular extension, one of which is elongate; and a transition sleeve 804F having a tubular extension and a rod/cord coupler shown integral with an elongate rod disposed opposite the tubular extension. Sleeves according to the invention may also be angled or lordotic, rather than parallel with such sleeves also including tubular extensions.
With particular reference to FIGS. 4-6, the bone screw assembly 1″ is illustrated with the sleeve 804B that includes a single cylindrical tubular extensions 830 (as do the sleeve 804E and the coupler 804F). The sleeve 804A includes two opposed extensions 830, while the sleeves 804C, D and E alternatively include one or more similar, but longer tubular extension 830′. The sleeve 804B (as well as the other sleeves 804A, C, D, E and F) further includes a body portion 834 generally sized and shaped for being received within the polyaxial bone screw receiver 10 and about a cord 806. A through bore 836 extends centrally through the body portion 834 and the tubular extension, the bore 836 being sized and shaped to slidingly receive the cord 806. The body portion 834 further includes a pair of opposed spaced radially extending flanges 837 and 838 with a partially cylindrical and partially planar body portion being located therebetween, the body portion 834 having a slightly enlarged or protruding portion or portions illustrated as partially cylindrical and partially planar surface portions 839, sized and shaped to closely fit within the inner arm surfaces of the bone screw receiver 10. The sleeve body portions 839 function to center the sleeve within the bone screw receiver 10 and also advantageously strengthen the sleeve, resulting in better load transfer. Each sleeve portion 839 has a bottom surface 839′ that engages and seats on top arm surfaces 144 of the compression insert 14 as best shown in FIG. 5. The body 834 with centering structure 839 further includes a lower or bottom surface portion 840 having a curved, U-shaped surface sized and shaped to closely cooperate and engage a curved saddle surface of the insert 14. The illustrated surface 840 is smooth, but it is foreseen that the surface 841 may be roughened.
Returning to the sleeve flange portions 837 and 838, the flange 837 differs from the flange 838 in part in that the flange 837 includes a small peripherally located U-shaped channel 837′ sized and shaped to receive a small extension portion or nub 821 of the spacer 816, best shown in FIG. 7, thus prohibiting sliding rotation of the spacer 816 with respect to the sleeve 804B (or any of the other sleeves, generally 804). With further reference to FIG. 7, the spacer 816 includes opposed planar side surfaces 817 and 818 with the nub 821 extending outwardly from the side 818. Thus, when the spacer 816 is cut to a desired sized, the user cuts the side 817 to leave the nub 821 in place. The spacer further includes a through bore 820 extending through the spacer and sized and shaped for slidingly receiving the cord 806. The bore 820 is located directly below the nub 821. The spacer further includes a curved top surface 822 as well as a curved bottom surface 823. Between the top and bottom surfaces are one or more planar and/or curved surfaces, resulting in the spacer 816 having a somewhat tubular and somewhat trapezoidal geometry with the through bore 820 located closer to the upper surface 822 than to the lower surface 823. The small torsion control extension or nub 821 extends outwardly from the side surface 818 and the top surface 822, sharing a portion of the top curved surface 822. The torsion control nub 821 further includes a substantially planar outer surface 824 that is parallel to the spacer outer surface 818 and opposed substantially planar side surfaces 825 that are sized and shaped to closely fit within surfaces of any of the sleeves 804 that include the flange 837 having the channel 837′ formed therein.
As illustrated in FIG. 1, each of the sleeves 804 have at least one flange 837 having the U-shaped channel 837′, the channel 837′ adjacent the tubular extension 830 or 830′. The flanges 838 located on the sleeves 804B, 804D and 804F do not include the channel 837′ and have in lieu thereof an inner solid concave or cylindrical surface 838′ sized and shaped to receive and be in close spaced relation with the closure top 18 or 18′. It is foreseen that in some embodiments, the flanges 837 and 838 may be reduced or the flange 838 may be eliminated as the centering of the sleeve with respect to the bone screw receiver 10 may be performed by the portion or portions 839. The illustrated flanges 837 and 838 are sized and shaped to provide for a close fit between each flange and the receiver 69 as best shown in FIG. 4. The body portion 839 as well as flange surfaces may be sized and shaped to be receivable by and frictionally fixed to a variety of monoaxial or polyaxial screw heads or receivers, including the receiver 10.
Each sleeve further includes a bore 843 formed in the body 834 at a seating surface 844 between the flanges 837 and/or 838, the bore 843 being transverse to and communicating with the through bore 836. The bore 843 is sized and shaped to receive the closure top 18 or 18′ therein. The sleeve 804B is shown with the closure top 18 in FIGS. 4-6, and thus, as best illustrated in FIG. 5, the top 18 does not extend down into the through bore 836, allowing for the cord 806 to slide freely there within. The closure top 18′, on the other hand (see FIGS. 2 and 6, e.g.) if inserted through the bore 843, extends into the sleeve 804A for frictionally gripping a cord 806 against an internal surface defining the through bore 836, and thus placing such cord 806 in fixed relation with the bone screw receiver 10, if desired.
The sleeves, generally 804, as well as the cord blocker 810 with set screw 812 may be made from a variety of inelastic materials, including, but not limited to metals, metal alloys, including cobalt chromium, and inelastic plastics including, but not limited to plastic polymers such as polyetheretherketone (PEEK), ultra-high-molecular weight-polyethylene (UHMWP), polyurethanes and composites, including composites containing carbon fiber and layers of different materials. The sleeves can also be made from elastic materials.
With reference to FIG. 1 and FIGS. 2-3, the sleeve and rod/cord coupler 804F further only includes a partial longitudinal bore 836′ substantially similar to the previously described bore 836, but not extending completely through an elongate solid rod portion 850. The closure top 18′ is illustrated as providing a fixed connection between the cord 806 and the rod 850. Alternatively a pin (not shown) may be used within an aperture or bore 851 disposed transverse to and communicating with the bore 836′, the through bore 851 sized and shaped to closely receive the cord holding pin (not shown). The pin, if used, extends completely through the cord 806, independently fixing the cord 806 to the sleeve 804F and integral rod 850. The rod portion 850 may be provided in a variety of lengths (or cut to length) to cooperate with one or more bone screws to provide a rigid support end to a dynamic assembly, such as the assembly 801 shown in FIG. 8.
With particular reference to FIG. 5, in operation, the sleeve 804B is shown with outwardly extending portions 839 directly engaged with and fixed between the closure top bottom surface 188 and the compression insert top arm surfaces 144 with the closure top rim 190 digging into the sleeve surfaces 844. Thus, a desirable even load distribution is created from the closure top 18 (or 18′), through the sleeve portions 837 and directly onto arm surfaces 144 of the compression insert 14, the compression insert 14 then in turn pressing evenly down upon the shank head 8, locking the head 8 into engagement with the retainer 12 and thus the entire bone screw assembly 1″.
It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown.
